1. Field of the Invention
This invention relates to a color signal converting circuit for image processing, particularly for image compression, in an apparatus which deals with image data such as a color printer and a color digital copying machine.
2. Discussion of the Background
Various color signal converting systems are conventionally known, and a converting system between an RGB signal system and a YIQ signal system as one of such conventional color signal converting systems is employed, for example, in an NTSC system. Such a converting system is disclosed, for example, in "New Edition of Color Science Handbook", edited by the Chromatics Society of Japan and published by Tokyo University Press, Feb. 25, 1980, p. 926.
First, expressions for converting signals of an RGB signal system into signals of a YIQ signal system are given below. EQU Y=0.30R+0.59G+0.11B (1) EQU I=0.74(R-Y)-0.27(B-Y) (2) EQU Q=0.48(R-Y)+0.41(B-Y) (3)
Expressions for converting signals of a YIQ signal system into signals of an RGB signal system reverse to the conversion expressions given above are given below. EQU R=Y+0.95I+0.62Q (4) EQU G=Y-0.27I-0.64Q (5) EQU B=Y-1.11I+1.72Q (6)
Here, R, G and B are signals of primary colors representing red, green and blue, respectively, while Y is a luminance signal and I and Q are color difference signals.
Circuit construction for such conversions as given by the conversion expressions (1) to (3) and the reverse conversion expressions (4) to (6) above is examined here.
The expressions (1) to (6) above each include a coefficient of a decimal number to a signal. Accordingly, it is necessary to deal with data in the form of floating-point numbers and include operators which execute additions, subtractions and multiplications.
Such a conventional color signal converting circuit as described above has several problems that circuit construction is complicated and the calculating speed is low because a large number of bits in binary number are required for the operation of a decimal number.